Global distribution of UV‐absorbing aerosols from Nimbus 7/TOMS data

TLDR

Minor sources of UV‑absorbing aerosols, such as volcanic ash and oil fires, are discussed. Global distributions of UV‑absorbing aerosols were derived from 340–380 nm radiance differences on Nimbus 7 TOMS (1979–1993), noting that boundary‑layer aerosols are hard to detect because of weak Rayleigh scattering and that cloud contamination can affect optical depth calculations. Time‑series analyses reveal that biomass burning and desert dust dominate annual UV‑absorbing aerosol injection, with major source regions in South America, Africa, the Sahel, Arabian Peninsula, India, Indonesia, Eastern China, and northeastern China; these aerosols can reach altitudes above 1 km, attain optical depths ≥0.1, and cover ~10 % of Earth’s surface, with significant year‑to‑year variability and evidence of anthropogenic dust contributions.

Abstract

Global distributions of UV‐absorbing aerosols are obtained using measured differences between the 340 and the 380 nm radiances from the Nimbus 7 Total Ozone Mapping Spectrometer (TOMS) for the years 1979–1993. Time series are shown for major sources of biomass burning and desert dust giving the frequency of occurrence and areal coverage over land and oceans. Minor sources of UV‐absorbing aerosols in the atmosphere are also discussed (volcanic ash and oil fires). Relative values of year‐to‐year variability of UV‐absorbing aerosol amounts are shown for major aerosol source regions: (1) central South America (Brazil) near 10°S latitude; (2) Africa near 0°–20°S and 0° to 10°N latitude; (3) Saharan Desert and sub‐Saharan region (Sahel), Arabian Peninsula, and the northern border region of India; (4) agricultural burning in Indonesia, Eastern China, and Indochina, and near the mouth of the Amazon River; and (5) coal burning and dust in northeastern China. The first three of these regions dominate the injection of UV‐absorbing aerosols into the atmosphere each year and cover areas far outside of their source regions from advection of UV‐absorbing particulates by atmospheric wind systems. During the peak months, smoke and dust from these sources are transported at altitudes above 1 km with an optical depth of at least 0.1 and can cover about 10% of the Earth's surface. Boundary layer absorbing aerosols are not readily seen by TOMS because the small amount of underlying Rayleigh scattering leads to a small signal. Significant portions of the observed dust originate from agricultural regions frequently within arid areas, such as in the Sahel region of Africa, especially from the dry lake‐bed near Lake Chad (13.5°N, 14°E), and intermittently dry drainage areas and streams. In addition to drought cycle effects, this suggests there may be an anthropogenic component to the amount of dust injected into the atmosphere each year. Detection of absorbing aerosols and calculation of optical depths are affected by the presence of large‐scale and subpixel clouds in the TOMS field of view.